To improve the printability of paper, the base paper made in a paper machine may be coated with a variety of coating formulations. The applied coat must be smooth and have a constant coat weight after it is applied to a moving web in a slurried form, and excess water is removed from the applied coat in dryers. The application of the coat may be divided into two steps, comprising the application of the coat and subsequent smoothing of the coat combined with metering of the applied coat.
A method used in the art is coat application to the paper web by means of a roll applicator, in which the web is passed in intimate contact over a rotating applicator roll. While the applicator roll is usually rotated along with the web at a speed lower than the web speed, counter-directional rotation and other types of differential speed arrangements are also known in the art. The coating mix is transferred onto the applicator roll surface by means of a metering device apparatus, or alternatively, the applicator roll is adapted to rotate in a coating mix pan. From the applicator roll surface, the coating mix is transferred to the surface of the moving web, and subsequent to application, the coat is further smoothed by doctoring away the excess coat with a doctor blade, levelling rod or air knife. The web to be coated can be pressed against the applicator roll by a variety of arrangements. In early coaters, a guide roll was placed to both sides of the applicator roll, and the web was kept against the applicator roll with help of the guide rolls. In such an assembly, the maximum usable applicator nip pressure between the web and the applicator roll is determined by the web tensile strength, which means that the maximum application pressure remains rather low. Such an arrangement is suited for low web speeds only, because the air layer travelling along with the web easily becomes entrained at high web speeds in the nip between the web and the applicator roll, thus causing surface defects in the coated web. Hence, this method is today used for coating webs other than paper in locations operated at sufficiently low web speeds. The web speed of such a low-speed coater may be increased marginally by injecting air to the backing side of the web, thus pressing the web against the applicator roll.
In paper manufacture, roll application occurs against a backing roll. In such coaters, the web is passed via a gap or nip formed between the applicator roll and the backing roll, whereby a higher nip pressure can be used. By virtue of the elevated nip pressure, the coating mix can be efficiently forced into the base paper in the roll applicator, which in conjunction with the swelling of the fibers and increase in the bulk volume facilitates well-controlled application of heavy coats. The applied coat weight is initially often many-fold in comparison with the final coat weight. Therefore, the deep penetration of the coat and large volume of doctored coating mix requires a great doctoring force to be controlled in the smoothing of the coat. Such a great doctoring force in turn imposes a heavy stress on the moist web and causes web breaks. It has been noted, that the major portion of web breaks occur at blade coaters, which means that an improvement in the reliable function of coaters would have a major impact on the production efficiency of paper manufacture and total availability performance of the paper manufacturing machinery.
At concurrent high web speeds, roll applicators are hampered by splashing of the coating mix, which is caused by the splitting of the coat film as the web exits the nip between the applicator roll and the backing roll. Here, a part of the coat film remains adhered to the web, while the other part sticks to the surface of the applicator roll, whereby the coat film will become extended between the web and the applicator roll forming droplets that are centrifugally thrown out from the nip. The larger the tangential exit angle of the web leaving the applicator roll, the more splashing will occur, because this condition is related to a more violent splitting of the coat film. Hence, concurrent coaters tend to use large-diameter rolls to reduce the tangential exit angle of the web leaving the applicator roll. Simultaneously, the larger-diameter of the applicator roll extends the application zone length and improves the application result. However, fabrication of such large-diameter applicator rolls to exact tolerances combined with good dynamic balancing is expensive, and, with the trend toward larger-diameter applicator rolls, the design and location of the coater in a factory building becomes ever more difficult simply due to the larger size of the coater machinery.
As roll applicators, however, can achieve heavy coats with good quality of the coat, reduction or elimination of the above-described shortcomings is most challenging.